The surface glycoproteins of enveloped viruses play critical roles in the initial events of viral infection, mediating virion attachment to cells and fusion of the viral and immune response in infected hosts. Envelope glycoproteins are also major targets for the anti-viral immune response in infected hosts. The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein consists of two noncovalently associated subunits, gp120 and gp41, that are generated by proteolytic cleavage of a precursor polypeptide, gp160. Luciw, P. A., In Fields Virology, Third Edition, B. N. Fields et al., eds., Lippincott-Raven Publishers, Philadelphia, pp. 1881-1952 (1996); Freed, E. O. et al., J. Biol. Chem. 270: 23883-23886 (1995). gp120 directs target-cell recognition and viral tropism through interaction with the cell-surface receptor CD4 and one of several co-receptors that are members of the chemokine receptor family. Broder, C. C. et al., Pathobiology 64:171-179 (1996); D'Souza, M. P. et al., Nature Med. 2:1293-1300 (1996); Wilkinson, D., Current Biology 6:1051-1053 (1996). The membrane-spanning gp41 subunit then promotes fusion of the viral and cellular membranes, a process that results in the release of viral contents into the host cell. It has not yet been possible to obtain a detailed structure for gp41, either alone or in complex with gp120.